I had O1 in 2009, and applied EB1A April 2012, and got the greencard this
August. For my personal experience. EB1A is definitely harder. I used the
same lawyer for both cases. O-1 is not necessary easier for preparation.
Both need take a long time(about 2 months) to collect evidence and reference
letters. The difference is more about the background. If you are very
confident and have a solid proof for your extraordinary, just go EB1A
directly. For approve time, I think O-1 will definitely faster. It took less
than 1 month for my application. My EB1A I140 took about 2 and a half
months, but I know somebody take half year for references.